1. Field of the Invention
The present invention relates to a process for preparing m,m'-dinitrobenzophenone which comprises reacting benzophenone with nitric acid in oleum.
2. Description of the Prior Art
When benzophenone is reacted with nitric acid, a product can be obtained containing an isomeric mixture of dinitrobenzophenones, including o,o'-, o,m'-, m,m'-, m,p'- and p,p'-dinitrobenzophenones, which, in turn, can be subjected to hydrogenation to obtain an isomeric mixture of the corresponding diaminobenzophenones. The latter mixture can be reacted with 3,4,3',4'-benzophenone tetracarboxylic dianhydride (BTDA) to obtain a polyimide resin. Although the m,m'-, m,p'- and p,p'-diamino benzophenones in said latter mixture will react satisfactorily with BTDA to form desired long-chain polyimide resins, the ortho-diaminobenzophenones will react with BTDA to a far lesser extent, resulting in a mixture of long and relatively short polyimide resins. This is believed to be the result of hydrogen bonding between an ortho amine hydrogen and the carbonyl, which reduces the basicity of the compound and renders the compound less reactive with BTDA. It would be highly desirable, therefore, to reduce the content of ortho-substituted isomers of dinitrobenzophenones and substantially increase the content of m,m'-dinitrobenzophenone in a mixture containing the same.